ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-12-5583-2012Radiative forcing estimates of sulfate aerosol in coupled climate-chemistry models with emphasis on the role of the temporal variabilityDéandreisC.1BalkanskiY.1DufresneJ. L.2CozicA.11Laboratoire des Sciences du Climat et de l'Environnement, UMR8212, IPSL, CEA-CNRS-UVSQ, Gif-sur-Yvette Cedex, France2Laboratoire de Météorologie Dynamique, LMD/IPSL, CNRS-UPMC, Paris, France26062012121255835602This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/12/5583/2012/acp-12-5583-2012.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/12/5583/2012/acp-12-5583-2012.pdf

This paper describes the impact on the sulfate aerosol radiative effects of
coupling the radiative code of a global circulation model with a
chemistry-aerosol module. With this coupling, temporal variations of sulfate
aerosol concentrations influence the estimate of aerosol radiative impacts.
Effects of this coupling have been assessed on net fluxes, radiative forcing
and temperature for the direct and first indirect effects of sulfate.
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The direct effect respond almost linearly to rapid changes in
concentrations whereas the first indirect effect shows a strong
non-linearity. In particular, sulfate temporal variability causes a
modification of the short wave net fluxes at the top of the atmosphere of
+0.24 and +0.22 W m<sup>−2</sup> for the present and preindustrial
periods, respectively. This change is small compared to the value of the net flux at the
top of the atmosphere (about 240 W m<sup>−2</sup>). The effect is more important in
regions with low-level clouds and intermediate sulfate aerosol
concentrations (from 0.1 to 0.8 μg (SO<sub>4</sub>) m<sup>−3</sup> in our model).
<br><br>
The computation of the aerosol direct radiative forcing is quite
straightforward and the temporal variability has little effect on its mean
value. In contrast, quantifying the first indirect radiative forcing
requires tackling technical issues first. We show that the preindustrial
sulfate concentrations have to be calculated with the same meteorological
trajectory used for computing the present ones. If this condition is not
satisfied, it introduces an error on the estimation of the first indirect
radiative forcing. Solutions are proposed to assess radiative forcing
properly. In the reference method, the coupling between chemistry and
climate results in a global average increase of 8% in the first indirect
radiative forcing. This change reaches 50% in the most sensitive regions.
However, the reference method is not suited to run long climate simulations.
We present other methods that are simpler to implement in a coupled
chemistry/climate model and that offer the possibility to assess radiative
forcing.